The present invention relates to an output level control circuit for use in a high frequency or radio frequency (RF) power amplifier, and more particularly to a control circuit of this kind suitable for use in the RF power amplifier of a transmitter in a communication system using RF signals having a plurality of intermittent and selectable output levels as in a TDMA radio communication system or a digital cellular mobile or Groupe Speciale Mobile (GSM) telephone system.
In order to maintain the transmitting power of a radio communication system at a prescribed level, there are needed an RF power signal level detecting circuit (hereinafter referred to simply as an RF detecting circuit) for detecting the level of the RF signal and a circuit responsive to the output of the RF detecting circuit for controlling a power amplifier. Especially in order to precisely keep the transmitting power at one of a plurality of prescribed levels over a broad temperature range, the RF detecting circuit is required to precisely detect, preferably without distorting linearity, the RF power level throughout the wide dynamic range of the transmitting power and, moreover, over a broad temperature range.
The RF detecting circuits described in "A Temperature Stabilized RF Detector with Extended Dynamic Range", R. J. Turner, 32nd IEEE Vehicular Technology Conference Record, May 23-26, 1982, pp. 231-242 and U.S. Pat. No. 4,523,155 (issued Jun. 1, 1985) are intended to satisfy the aforementioned requirements, and are applicable not only to such intermittent RF signals as mentioned above but also to consecutive RF signals. Any such a conventional circuit requires two diodes including a detecting diode responsive to the envelope of RF signal for generating an output and a temperature compensating diode having the same characteristics as and thermally coupled with the detecting diode. In that circuit, the two diodes are provided with the same forward bias voltage, and the difference between the load voltage of the detecting diode and that of the temperature compensating diode is used as the detection output of the RF power level. Since this detection output is obtained by subtracting the forward voltage of the temperature compensating diode from that of the detecting diode, any variation in the latter with a change in temperature is cancelled by a corresponding variation in the former. Therefore, the prior art RF detecting circuit can generate detection output voltages substantially proportional to the RF signal level without being affected by temperature fluctuations.
It is difficult, however, for such an RF detecting circuit to keep the biases of the two diodes at the same level over a broad temperature range because uneven characteristics of the circuit elements used, especially differences between the detecting and temperature compensating diodes in forward voltage and temperature characteristic and uneven resistances of bias voltage supplying resistors and load resistors, are inevitable. Moreover, the two diodes are usually arranged as separate parts to enable them to sensitively detect RF signals, and accordingly it may be difficult to install them thermally coupled in locations having the same temperature conditions. For these reasons, there remains in the detection output voltage a variant due to temperature variations. Further in order to compensate for the uneven characteristics of circuit elements, fine adjustments are required, including the offsetting of the bias voltages of the two diodes. Besides these problems, because of the need to constitute the two diodes by separate parts, it is difficult to wholly integrate the RF detecting circuit and the power amplifier control circuit for the purpose of reducing their cost and size.